1. Field of the Invention
The present invention relates to a method of preventing interference of adjacent frequencies in a cellular system, and more particularly to a method of preventing interference of adjacent frequencies in a cellular system that may be caused by adjacent carrier frequencies for uplink channels used by the independent cellular systems.
2. Description of the Related Art
In general, a specified frequency band is allocated to a cellular system, and a plurality of carrier frequencies which different frequencies are predetermined within this frequency band. A mobile station establishes a channel between itself and a base station in a service area using the carrier frequencies to perform two-way communication therebetween. The base station and the mobile station are designed so as to control leakage power into frequency bands adjacent to both sides of the used carrier frequency as little as possible. However, the leakage power can not be fully suppressed, so that an interference signal power is supplied to a signal of an adjacent frequency at predetermined rate. In one cellular system, each of the mobile stations establishes a channel between itself and the base station closest to it to perform communication. In an uplink channel which is a channel established from a mobile station to a base station, the transmission power of the uplink channel for transmitting a signal from the mobile station to the base station is controlled such that the difference between received powers of the signals from the mobile station, received by the base station may not be large. For this reason, an interference signal power supplied from a signal of an adjacent frequency becomes smaller than a desired signal power. On the other hand, in a downlink channel which is a channel established from a base station to a mobile station, if a transmission power of the base station is constant, an interference signal power from signals transmitted on a frequency adjacent to that of a desired wave signal reaches the mobile station through the same transmission path to be attenuated similarly. Accordingly, the interference signal power from the signal having a frequency adjacent to that of the desired wave signal is smaller than the desired signal power. Thus, in one cellular system, such interference of adjacent frequencies may not pose a serious problem.
However, the interference of adjacent frequencies may pose a serious problem when another cellular system having an allocated frequency band adjacent to the frequency band of the particular cellular system establishes a base station in the same service area and communicates with a mobile station.
As shown in FIG. 1, for example, when mobile station 221 located near base station 261 belonging to the other cellular system establishes a channel and communicates with base station 211, it sends a signal with a power sufficiently large for reception to base station 211 remote from mobile station 221, so that the signal is also received with a large power by base station 261. When mobile station 271 establishing a channel with base station 261 controls the transmission power so that the received power is not too large at base station 261, only the received signal from mobile station 221 may be exceedingly large. At this time, when a signal of a carrier frequency adjacent to a carrier frequency used for the transmission of mobile station 221, is used for the transmission by mobile station 271 the interference of adjacent frequencies of the transmitted signal of mobile station 221 will be large at base station 261, thereby deteriorating a receiving quality of the signal from mobile station 271.
One of solutions to such a problem is to make an interval between carrier frequencies used in different cellular systems wider than that between the carrier frequencies used in the same cellular system. Because the spacing between the carrier frequencies among the cellular systems becomes wider, this solution is disadvantageous in reducing utilization efficiency of frequency bands. However, in a cellular system using many narrow band signals, the spacing between the carrier frequencies is small as compared with the total of the frequency band, so that the reduction in the utilization efficiency is not so serious. On the other hand, in a cellular system in which a small number of wide band signals in the same frequency band are used, the spacing between the frequency bands is large as compared with the total of the frequency bands, thus significantly reducing the utilization efficiency.
An approach to prevent interference between the cellular systems, has been proposed wherein prior to the start of transmission of an uplink channel by the use of a certain carrier frequency at a mobile station, interference signal power of a carrier frequency of a downlink channel used in conjunction with the carrier frequency of an uplink channel is measured, and when an interference signal power from a carrier frequency of an adjacent frequency is large, a carrier frequency other than that carrier frequency is used. However, in this approach, a certain combination of a carrier frequency of an uplink channel and a carrier frequency of a downlink channel must be used.
Furthermore, in a cellular system which adopts a code division multiple access method (CDMA) as a radio access method, despite a certain combination of carrier frequencies for an uplink channel and carrier frequencies for a downlink channel, signals to be transmitted to many mobile stations are multiplexed onto a signal of one carrier frequency of this reason, a received power of signals transmitted to other mobile stations using the same carrier frequency is received with an interference signal power from a signal of an adjacent carrier frequency mixed. Thus, despite the measurement of the received power, the interference signal power from the adjacent carrier frequency can not be obtained. Accordingly, with this method, it is impossible to prevent the interference from the adjacent carrier frequency.
Furthermore, since a transmitter in the mobile station has typically a simpler construction as compared with the base station, it has sometimes a low suppression of a leakage power into an adjacent carrier frequency. Therefore, when a certain mobile station is located near a base station belonging to another cellular system and the mobile station performs a transmission using adjacent carrier frequencies for an uplink channel as well as for a downlink channel, the mobile station is able to continue the communication with a good quality without being applied with an interference of adjacent frequencies because of a low leakage power from transmitting signals of the base station to the adjacent carrier frequency. However, because of a large leakage power from transmitting signals from the mobile station to an adjacent carrier frequency. A mobile station may cause an one-sided interference obstacle to the base station.
Although such interference of adjacent frequencies presents no problem when starting a communication, it may be a problem during the communication because as a mobile station may approach a base station belonging to another cellular system, as it moves during the communication.
It is an object of the present invention to provide a method of preventing interference of adjacent frequencies which is capable of preventing a degradation of communication quality due to an interference between carrier frequencies adjacent to each other, without widening the frequency interval of signals used in different cellular systems.
When two cellular systems utilizing frequency bands adjacent to each other for an uplink channel, are provided in a same service area, and a signal for the uplink channel used by a mobile station of one cellular system may cause interference of adjacent frequencies to a mobile station of the other cellular system, the method of preventing an interference of adjacent frequencies in a cellular system according to the present invention is used for preventing an interference to a base station of another cellular system from a signal of an uplink channel used by a mobile station of one cellular system.
To achieve the foregoing object, in the method of preventing interference of adjacent frequencies of a cellular system according to the present invention, a mobile station of the one cellular system measures a received power of transmitting signals transmitted from a base station of the other cellular system present around the mobile station of one cellular system. If the measured value is larger than a threshold value, the mobile station selects and uses a carrier frequency not adjacent to a carrier frequency as a carrier frequency of an uplink channel used in the other cellular system.
The mobile station of one cellular system measures a received power of transmitting signals transmitted from a base station of another cellular system, whereby a amount of the interference of adjacent frequencies given to the base station of the other cellular system by the mobile station of one cellular station is measured. If the interference of adjacent frequencies is larger than a threshold value, it is decided that the amount of the interference of adjacent frequencies received by the base station of the other cellular system exceeds an allowable value. When the mobile station selects a carrier frequency of the uplink channel, the mobile station selects and uses a non-adjacent carrier frequency as the uplink channel, in order to prevent the interference of adjacent frequencies to the base station. As a result, the interference of adjacent frequencies to the base station by the mobile station can be prevented.
Further, according to this embodiment of the present invention, the threshold value is determined based on the amount of suppression of leakage power from an adjacent frequency of the mobile station.
According to another embodiment of the present invention, the threshold value is determined based on a received power of transmitting signals transmitted from a base station of the one cellular system.
According to another embodiment of the present invention, the threshold value is determined based on a difference between a transmission power of transmitting signals transmitted from a base station in which a channel is established with a mobile station, and a transmission power of transmitting signals transmitted from a base station of the other cellular system present around the mobile station.
In another method of preventing interference of adjacent frequencies in a cellular system according to the present invention, when a mobile station of the one cellular system receives a downlink channel continuously, the mobile station preferentially uses a carrier frequency not adjacent to a carrier frequency of a signal of an uplink channel used in the other cellular system. When the mobile station of one cellular system intermittently receives the downlink channel, the mobile station measures a received power of transmitting signals transmitted from a base station of another cellular system present around the mobile station. If the measured value is larger than a threshold value, when the mobile station selects a carrier frequency of a signal of an uplink channel to be used, the mobile station selects and uses a carrier frequency not adjacent to the carrier frequency of the signal for the uplink channel used in the other cellular system.
In another method of preventing interference of adjacent frequencies according to the present invention, the mobile station of one cellular system measures a received power of transmitting signals transmitted from the base station of the other cellular system present around the mobile station, and the mobile station obtains an upper limit value of a transmission power from the measured value, which makes the interference of adjacent frequencies equal to or less than an allowable value. If the upper limit value of the transmission power is equal to or less than the transmission power value at the time of aiming a SIR as a target value, when the mobile station selects a carrier frequency of a signal for the uplink channel, the mobile station selects and uses a carrier frequency which is not adjacent to the carrier frequency of the signal for the uplink channel used in another cellular system.
In another method of preventing interference of adjacent frequencies according to the present invention, the mobile station of one cellular system measures a received power of transmitting signals which is transmitted from the foregoing base station of another cellular system around the mobile station. If the measured value is larger than a first threshold value, when the mobile station selects a carrier frequency of a signal for the uplink channel used, the mobile station selects and uses a carrier frequency which is not adjacent to the carrier frequency of the signal for the uplink channel used in another cellular system. If the measured value is equal to or less than a second threshold value which is smaller than the first threshold value, when the mobile station selects a carrier frequency of a signal of an uplink channel used, the mobile station preferentially uses a carrier frequency adjacent to the carrier frequency of the signal for the uplink channel used in another cellular system.
According to an embodiment of the present invention, the first threshold value is determined based on a difference between a transmission power of transmitting signals which is transmitted from a base station in which a channel is established with a mobile station and a transmission power of transmitting signals which is transmitted from a base station of another cellular system around the foregoing mobile station.
In another method of preventing interference of adjacent frequencies of a cellular system according to the present invention, the mobile station of one cellular system estimates a movement speed. If the movement speed is larger than a certain threshold speed, when the mobile station selects a carrier frequency of a signal of an uplink channel used, the mobile station selects and uses a carrier frequency which is not adjacent to a carrier frequency of a signal for the uplink channel used in another cellular system.
According to another embodiment of the present invention, the mobile station uses information concerning a central frequency of a signal of a downlink channel used by a base station of another cellular system around the mobile station, the information being informed from a base station belonging to the same cellular system of the mobile station, and measures a received power of transmitting signals transmitted from the base station of another cellular system around the mobile station, whereby the received power of the transmitting signal transmitted from the base station is measured.
According to another embodiment of the present invention, the mobile station uses information concerning a central frequency of a signal of a downlink channel that is being used by a base station of another cellular system around the mobile station, the information being previously possessed by the base station, and measures a received power of transmitting signals transmitted from the base station of another cellular system around the mobile station, whereby the mobile station measures the received power of the transmitting signal transmitted from the base station.
In another method of preventing interference of adjacent frequencies of a cellular system according to the present invention, the foregoing mobile station of one cellular system measures a received power of transmitting signals transmitted from a specified base station other than base stations of another cellular system in which a carrier frequency adjacent to a carrier frequency of a signal of an uplink channel used in one cellular system is not determined, among base stations of another cellular system around the foregoing mobile station of one cellular system. If the measured value is larger than a threshold value, when the mobile station selects a carrier frequency of a signal of an uplink channel, the mobile station selects and uses a carrier frequency which is not adjacent to a carrier frequency of a signal of an uplink channel used in another cellular system.
In another method of preventing interference of adjacent frequencies of a cellular system according to the present invention, a mobile station of one cellular system measures a received power of transmitting signals transmitted from a base station of another cellular system. If the measured value is larger than a threshold value, when the mobile station selects a carrier frequency of a signal of a downlink channel used, the mobile station selects and uses a carrier frequency which is not adjacent to a carrier frequency of a signal of a downlink channel used in another cellular system.
According to an embodiment of the present invention, by measuring a received power of a perch channel in transmitting signals transmitted from a base station, the mobile station measures a received power of the transmitting signal transmitted from the base station.
According to another embodiment according to the present invention, in the case where as a carrier frequency of a signal of an uplink channel to be used, the mobile station selects and uses a carrier frequency which is not adjacent to a carrier frequency of a signal of an uplink channel that is being used in another cellular system, the mobile station selects and uses a carrier frequency which is not adjacent to a carrier frequency of a signal of a downlink channel that is being used in another cellular system, as a carrier frequency of a signal of a downlink channel used.
According to another embodiment of the present invention, when a mobile station starts to perform communication with a base station, the mobile station measures a received power of a carrier frequency directed by the base station, and informs the base station of the measurement result of the received power, using a carrier frequency which is not adjacent to a carrier frequency of another cellular system. Upon receipt of the measurement result from the mobile station, if the received power measured is larger than a threshold value, the mobile station selects a non-adjacent frequency. If the received power measured is smaller than the threshold value, the mobile station arbitrarily selects a carrier frequency which can be used, among allocated carrier frequencies.
In another method of preventing interference of adjacent frequencies of a cellular system according to the present invention, a mobile station of one cellular system measures a received power of transmitting signals transmitted from a base station of another cellular system present around the mobile station, and sends the measured value to a base station in which a channel is established with the foregoing mobile station. If the measured value is larger than a threshold value, when the base station selects a carrier frequency of a signal of an uplink channel to be used by the mobile station, the base station to which the measured value is transmitted selects a carrier frequency which is not adjacent to a carrier frequency of a signal of an uplink channel that is being used in another cellular system, and informs the foregoing mobile station of it. Then, the mobile station uses the informed carrier frequency as the uplink channel.
In the present invention, a mobile station of one cellular system measures a received power value of transmitting signals transmitted from a base station of another cellular system, and reports the measurement result to a base station of one cellular system, in which a channel is established with the mobile station of one cellular system, whereby the base station of one cellular system decides a magnitude of an interference of adjacent frequencies which is given to a base station of another cellular system by the mobile station of one cellular system. If the magnitude is larger than a threshold value, the base station of one cellular system decides that the amount of the interference of adjacent frequencies given to the base station of another cellular system from the mobile station of one cellular system is more than an allowable value. When the mobile station selects a carrier frequency of an uplink channel used, the mobile station selects a non-adjacent carrier frequency as uplink channel in order to prevent the interference of adjacent frequencies given to the base station of another cellular system, and the mobile station uses the non-adjacent carrier frequency.
Therefore, the interference of adjacent frequencies which is to be given to the base station of another cellular system by the mobile station of one cellular system can be prevented.
The above and other objects, features and advantages according to the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples according to the present invention.